1. Field of the Invention
This invention relates to laminating a metal foil or sheet to a thermoplastic polymer substrate at a preselected temperature and to the articles produced thereby. More specifically, this invention relates to the manufacture of printed circuits, and especially to printed circuits produced by subtractive processes on engineering thermoplastic base materials.
2. Description of the Prior Art
High temperature engineering thermoplastics, such as polysulfones, polyethersulfones, polyarylsulfones and, polyetherimides, having excellent dielectric properties. Many processes have been proposed for producing printed circuits on such thermoplastic materials to take advantage of their dielectric insulating properties.
Printed circuits have been produced on high temperature engineering thermoplastics by the semi-additive process. In the semi-additive process, an extruded thermoplastic sheet is provided with holes, annealed, adhesion promoted, electrolessly plated with a thin layer of copper, and printed with a negative resist image. The exposed circuit pattern is electroplated with copper, and then the resist and underlying thin copper film is removed.
Printed circuits also have been produced on these adhesion promoted thermoplastic materials by the additive process using the imaging processes of Polichette et al., U.S. Pat. Nos. 3,722,078; 3,930,963; 3,959,547 and 3,994,727.
Over ninety percent of all printed circuits produced in the United States are manufactured by subtractive processes. Most subtractive processing starts with a copper clad base material. Subtractive printed circuit manufacturing facilities do not have additive or semi-additive manufacturing capabilities.
Since 1975, the printed circuit industry has sought high temperature thermoplastics as base materials. See, e.g., Modern Plastics, June 1975, pp. 52-54, "New Day Dawns for Circuit Boards". Widescale manufacture to fill this need has not been possible due to the lack of suitable copper clad, high temperature, thermoplastic base materials.
Naegle, IPC-TP-319, the Institute for Interconnecting and Packaging Electronic Circuits, Evanston, Illinois, April 1980, "Polysulfone: Its Microwave Properties", describes two types of copper clad polysulfone. In one type, copper foil is directly bonded to a polysulfone sheet. However, Naegle does not describe how this direct bonded copper clad polysulfone was prepared, and it has not been made available commercially. In the second type, copper foil is bonded to the polysulfone substrate by an intermediate layer of prepreg, epoxy resin impregnated glass cloth. The adhesion of the copper foil to the base material is superior in this second type, but all other properties are described as inferior. The second type was commercially available, but it was subsequently withdrawn from the market.
Going, "Polyetherimide: A New High Performance Polymer for Printed Circuit Applications", October 1982 Meeting American Society for Electroplated Plastics, describes the formation of copper clad polyetherimide substrates. Going recommends laminating copper foil to a polyetherimide base material at temperatures of 260.degree. C. to 288.degree. C. and pressures of 3.5 MPa (500 psi) or less. The copper-polyetherimide base material produced by this procedure is not completely satisfactory, due to the stress and flow of the resin during lamination. The laminate has a tendency to warp after subtractive printed circuit processing and to have thickness variations from place to place on the base material.